Numerous applications exist where it is necessary or desirable to pump two or more incompatible materials to a common destination through a restricted circular opening or where it is necessary to provide a supply and a return through such an opening. The most obvious examples occur in deep well drilling and examples in air conditioning systems, hot and cold water delivery systems and in hydraulic control systems are readily apparent. Couplings for such coaxial piping systems must provide a tight seal for separation of fluids and must be easily and quickly broken and resealed. Preferably, there should be a minimal pressure drop across the coupling when compared to straight pipe.